1. Field of the Invention
This invention relates to the use of a plasma gas stream to mill materials. Particularly, the invention relates to the use of the plasma to drill small holes, such as those used in printed circuit boards, including multi-layer printed circuit boards.
In the formation of printed circuit boards, it is often desirable to mill holes having a size smaller than that obtainable by using mechanical milling techniques such as a rotating tool bit. By carefully controlling the size of the holes, it is possible to reduce the size of micro-circuits, particularly when the micro-circuits are produced with multi-layer circuit technology.
The drilling of small holes is expected to be important in the fabrication of mechanical components which must be made to precision tolerances. Accordingly, this invention will be used to mill a wide variety of materials for a wide variety of applications in addition to the Kapton (TM, E. I. du Pont de Nemours & Co.) substrates of printed circuit boards.
2. Description of the Prior Art
Various techniques have been advanced for milling through conductive and insulative substrates in order to create via holes and the like. The simplest technique involves mechanical milling. The primary difficulty with this technique is the size limitation, particularly with respect to the minimum diameter of the hole which may be drilled or otherwise milled away. It is often difficult, particularly in multi-layered devices, to ascertain that a mechanically-drilled hole is smooth enough (with no splintered conductors) to insure that the layers are maintained electrically separate except for where the desired connections are to be made.
In one prior art technique, using an electron beam, the device to be etched is placed in a high vacuum and a beam of electrons is directed to the site to be etched. In addition to the necessity of providing a sizable vacuum chamber, ion etching is fairly time consuming. Because of the desirability of increasing the number of operations which can be performed with one pump down, large turntables or other coonveyer devices are used to feed sequential work pieces to a target area in the electron beam apparatus. These turntables require either precise guidance or precise alignment of the substrates to be drilled. This is usually accomplished by a robot. This technique has a disadvantage of being time consuming in its operation.
An ion etching technique has been developed which uses reactive gas at low pressures. The gas is energized into a plasma by R.F. energy. An ion beam is then formed by magnetic fields which cause the gas molecules to form an ion beam. The ion beam passes through a mask, after which it accomplishes a fine etch on a circuit device. Gas plasma devices have been developed in order to provide an etching of various films, such as polycrystaline silicon. In these devices, energy is used to produce a plasma which is then used to etch a workpiece.
Other techniques for milling include various chemical and photo-chemical etching techniques. These techniques are usually limited to the etching of metallic substrates. In the case of multi-layer circuits, it is necessary to sequentially etch and dissolve each of several of the plural layers, often after protecting the upper layers which have already been etched in previous steps. Additionally, chemical etching is usually a fairly slow, as well as tedious, technique, thereby raising the cost of fabrication of multi-layer miniature circuit boards. Since multi-layer circuit boards are used to reduce the necessity of hand wiring electronic circuits and to increase the level of automation in assembly, it is counterproductive to require that elaborate or time-consuming techniques be used in the fabrication of such multi-layer circuit boards.